Cold-atom sensors have already demonstrated excellent performance in the measurement of time (clocks) and of gravitational fields, of accelerations and of rotations. Their operating principle is recalled below.
The cold-atom sensors employed in atomic clocks use a gaseous cloud of atoms in a vacuum chamber. These atoms are such that they have two what are called “hyperfine” atomic levels that are separated in frequency by a quantity δf0, which is of the order of a gigahertz, with δf0=ω0/2π, that is very stable and very well known. These atoms are typically atoms of rubidium 87, for which δf0=6.834 GHz.
These atoms are initially in one of two fundamental states. A pulse generated by an oscillator with an angular frequency ω (called the π/2 pulse) is applied to these atoms, then a certain time tf is waited, then a second pulse that is identical to the first is applied. The atoms are then distributed between the two “hyperfine” fundamental states and the measurement of the respective populations of atoms allows the quantity ω0−ω to be calculated, this allowing the oscillator to be locked on the atomic oscillation.
To measure an acceleration, the two fundamental states are separated spatially with magnetic fields. The measurement of the two populations of atoms subjected to the acceleration is proportional to:
s=ω0−ω−m·a·d/ℏ, where a is the acceleration to which the cloud is subjected, d the spatial separation of the fundamental states, ℏ the reduced Planck constant and m the mass of the atom.
The acceleration is for example measured by measuring a movement of the fringes of a Ramsey interferometer in the frequency domain (typically as in a Mach-Zehnder interferometer in optics).
A sensitivity to the Sagnac effect is necessary to measure a speed of rotation. To achieve this, the atoms travel a closed path containing an area, this path being travelled in opposite directions by the two internal states.
To produce a cold-atom inertial sensor exploiting the effects described above, the measurement includes three main phases, a cooling phase, a pumping phase and a detecting phase.
By way of nonlimiting example, the principle of the sensor is described below for rubidium 87 atoms, rubidium 87 being the most commonly used type of atom, but other alkali atoms, such as atoms of rubidium 85 (δf0=3.0 GHz) cesium (δf0=9.2 GHz), sodium (δf0=1.7 GHz) or potassium 40 (δf0=1.3 GHz), having the same type of atomic structure may be used.
FIG. 1 illustrates the main atomic levels of interest of rubidium.
The two fundamental levels F=1 and F=2 are separated by δf0=6834 MHz. The excited levels F′=0, 1, 2 or 3 are obtained by optical excitation in the vicinity of 780 nm, and are separated from one another by amounts comprised between 50 and 300 MHz.
FIG. 2 illustrates the frequencies required in the three aforementioned phases.
In the cooling phase, a three-dimensional magneto-optical trap is formed. To do this, the first laser L1, which is called the cooling laser, is regulated to a frequency fRefroid that is slightly below an excited level by a quantity ε1, typically comprised between a few MHz and about one hundred MHz. The atoms draw on their kinetic energy to reemit at the excitation frequency and slow down. Preferably, the frequency of the laser L1 is decreased by about one hundred MHz between the start and the end of the cooling phase. In practice, the initial beam of the laser is split into 3 beams that are used to slow the three directions, for example using a cube corner (see FIG. 8).
During the cooling, to get all the atoms to the same fundamental level, a second laser L2, called the “repump” laser, of frequency fRepomp, is used to optically pump the atoms to one of the two hyperfine levels, for example F=2. The choice of the levels is based on the spectral selection rules of the atom in question.
Once the atoms have been cooled, the optical pumping second phase places all the atoms into a given Zeeman sub-level of the fundamental state F=2. A magnetic field is used to split the degeneracy of the various levels via the Zeeman effect. The cooling laser L1 is reused as pump laser (it then illuminates the atomic cloud in a different direction to during the cooling) for the pumping, which requires to be at a frequency fpomp that is below a set transition by a quantity ε2 of about 160 to 260 MHz, and the second “repump” laser L2 is also used to bring all the atoms to the same fundamental level.
In a detecting third phase (after a certain interferometry time) only the laser L1 is used, here as detection laser, with a frequency fdet adjusted to an atomic resonance.
In a cold-atom inertial sensor according to the prior art such as illustrated in FIG. 3, the frequency f1 of the laser L1 and the frequency f2 of the laser L2 are each very precisely stabilized on an atomic frequency of rubidium using control loops BAet1 and BAet2 that each use a rubidium saturated absorption cell Cell1 and Cell2. The frequency of the laser L1 is stabilized from the fundamental state F=2 and the frequency of the “repump” laser L2 from the fundamental state F=1. The right frequencies for the two lasers L1 and L2 and for each measuring (cooling, then optical pumping then detecting) phase are obtained using a very complex optical system and at least 5 acousto-optical modulators. The AOM3 serves in the stabilization of the laser L1 on an atomic transition, and the AOM4 serves in the stabilization of the laser L2 on an atomic transition. The AOM7 serves in the realization of the cooling phase, the AOM6 serves in the realization of the optical pumping phase and the AOM1 serves in the detecting phase.
Existing solutions for producing the system required to cool and to manipulate cold atoms are of two types:
the entire system may be produced with fiber-based components: the assembly then fits into a bay of 1 m×1 m×2 m height, such a bulk being incompatible with any on-board application; or
micromechanical technology, similar to the laser system of the Pharao atomic clock produced by the company Sodern, may be used: this solution is more compact than the preceding one but extremely costly because of the need to align and bond a very great number of miniature optical components.
One aim of the present invention is to mitigate the aforementioned drawbacks by providing a simplified laser-source assembly that is compatible with an integrated optic realization.